An intake manifold of an internal combustion engine which is a nonmagnetic member, and which is made of synthetic resin is provided with an intake air flow control valve such as a tumble control valve and a swirl control valve, and a position sensor arranged to sense a variation of magnetic flux of a magnet attached to a rotation member of this intake air flow control valve, and thereby to sense an opening degree of this intake air flow control valve (cf. a patent document 1). In this assembly structure of the position sensor, for example, a magnet holding portion arranged to hold the magnet is inserted into a hollow portion of the intake manifold through which the rotation member is inserted, from the opening end portion side of the hollow portion, so that the magnet holding portion is assembled to one end of the rotation member. The opening end portion of this hollow portion is closed by a cover which is a different member in a closing state. The position sensor is tightened and fixed to a portion of the intake manifold which confronts the magnet, by using a fixing tool such as screw and bolt.